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From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Indexed list
Date: Fri, 15 Aug 2003 13:01:31 +0000 (UTC)
Organization: IKS GmbH Jena
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Date: 2003-08-15T13:01:31+00:00
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* Alex Crivat wrote:
> Is there a way of doing this in Ada using operators or should I use a
> custom function;

You should use a custom function, but if you really need this, there is a
trick.  The '()' syntax is used in two contexts.  First as an array
subscript, as you like to recognise it.  And second as a function call.
Using access variables to function calls the same syntax applies.

So you have to set a variable either to an array or a function. Because a
variable can't be of type function, use access to function instead. You only
need a slight modification to set the initial value.

I'll provide you the short and the generic solution:
------------------------------------------------------------------------
with Ada.Text_IO;

procedure t is
   function Get(i : Natural) return Natural is
   begin
      return i + 4;
   end Get;
   
   type List is access function (i : Natural) return Natural;
   
   a : Natural;
   f : List := Get'Access;
begin
   a := f(3);
   Ada.Text_IO.Put_Line("a =" & Natural'Image(a));
end t;
------------------------------------------------------------------------
with Ada.Text_IO;

procedure t is
   generic
      type Item (<>) is limited private;
      type Offset (<>) is limited private;
      with function Get(i : Offset) return Item is <>;
   package GenList is
      type List is access function (i : Offset) return Item;
      Current : constant List;
   private
      function MyGet (i : Offset) return Item renames Get;
      Current : constant List := MyGet'Access;
   end GenList;
   
   function Get(i : Natural) return Natural is
   begin
      return i + 4;
   end Get;
   
   package MyList is new GenList(Natural, Natural);
   use MyList;
   
   a : Natural;
   f : List := Current;
begin
   a := f(3);
   Ada.Text_IO.Put_Line("a =" & Natural'Image(a));
end t;
------------------------------------------------------------------------

Have fun.